Projection and electronic display systems are widely used to display image content. In the case of projection systems, whether the traditional film based systems, or the newer electronic systems, light from a light source (typically a lamp) is directed to an image modulation element (such as film or one or more spatial light modulators) that imparts image data to the transiting light. Typically the film or light modulator arrays are then imaged to the display surface or screen.
As another aspect, there is growing interest in high-quality projection systems that display 3 dimensional (3D) or perceived stereoscopic content in order to offer consumers an enhanced visual experience. Historically, stereoscopic content was projected in theaters using film media, such that two sets of films are loaded to two separate projection apparatus, one for each eye. Left- and right-eye images are then simultaneously projected using polarized light, where one polarization is used for the image presented to the left eye; light of the orthogonal polarization is then used for the image presented to the right eye. Audience members wear corresponding orthogonally polarized glasses that block one polarized light image for each eye while transmitting the orthogonal polarized light image.
Recently, electronic or digital cinema projectors that provide stereo projection have been commercialized. In particular, projectors based on the Digital Light Processor (DLP) or Digital Micro-mirror Device (DMD), developed by Texas Instruments, Inc., Dallas, Tex., are used in theatres in both stereo and non-stereo versions. DLP devices are described in a number of patents, for example U.S. Pat. Nos. 4,441,791, 5,535,047 and 5,600,383 (all to Hornbeck).
FIG. 1A shows a simplified block diagram of a projector 100 that uses DLP spatial light modulators. A light source 50 (typically a xenon arc lamp) provides polychromatic unpolarized light into a prism assembly 55, such as a Philips prism, for example. Prism assembly 55, which is shown as a Philips prism, splits the polychromatic light into red, green, and blue component wavelength bands and directs each band to a corresponding spatial light modulator (SLM) 170r, 170g, or 170b. Prism assembly 55 then recombines the modulated light from the spatial light modulators 170r, 170g, and 170b and provides this unpolarized light to an imaging lens 200 for projection onto a display screen or other suitable surface. DLP-based projectors have demonstrated the capability to provide the necessary light throughput, contrast ratio, and color gamut for most projection applications from desktop to large cinema. Alternately, liquid crystal devices (LCDs), which modulate light by altering polarization states of transiting light, can be used instead of DLP devices, with comparative benefits of higher resolution and larger device size, but greater difficulty in delivering contrast, contrast uniformity, and color uniformity for the projected image.
Conventional methods for forming stereoscopic images from these SLM-based projectors (DLP or LCD) use either of two primary techniques to distinguish between the left- and right-eye content. One less common technique, utilized by Dolby Laboratories, for example, uses color space separation, such as described in U.S. Patent Application Publication 2007/0127121 by Maximus et. al. Filters are used in the white-light illumination system to momentarily block out portions of each of the primary colors for a portion of the frame time. The appropriate color adjusted stereo content that is associated with each eye is then presented to each modulator for the eye. The viewer wears a corresponding filter set that similarly transmits only one of the two 3-color (RGB) spectral sets. Color space separation avoids problems in handling polarized light from the projector, at the screen, and with the viewer's glasses, but light inefficiencies and the cost of the glasses are problematic.
The second method for forming stereoscopic images uses polarized light. For example U.S. Pat. No. 6,793,341, by Svardal et al. describes a method in which the two orthogonal polarization states are provided by separate spatial light modulators and projected simultaneously onto the screen, which typically has properties to preserve the polarization states of the reflected light. The viewer wears polarized glasses with polarization transmission axes for left and right eyes orthogonally oriented with respect to each other. Although this arrangement offers efficient use of light, it can be an expensive configuration.
Another approach, commercialized by Real-D, Beverly Hills, Calif., uses a conventional projector modified to modulate alternate polarization states that are rapidly switched from one to the other. In particular, as shown in FIG. 1A, a DLP projector is modified to have a polarizer and polarizer switching device placed in the output path of the light, such as at a position 90 indicated by a dashed line in FIG. 1A. The polarization switcher is required, since DLP projectors output modulated, but unpolarized light. This output is unpolarized because unpolarized light sources (lamps) are used, and the typical DLP device windows are depolarizing (due to stress induced birefringence). An achromatic polarization switcher, such as that of U.S. Pat. No. 7,528,906 to Robinson et al. can be placed at position 90 after the polarizer. A switcher of this type (the ZScreen™) alternately rotates polarized light between two orthogonal polarization states, such as linear polarization states, to allow the presentation of two distinct images, one to each eye, while the user views the projected image with polarized glasses.
Because the polarization contrast specifications for the polarizer are modest (˜50:1) as a trade-off to boost polarizer efficiency, image crosstalk between the left-eye and right-eye images can occur. This can cause viewers to experience ghost images, for example such that the left eye not only sees a bright left-eye image but a dim right-eye image. Real-D provides a variety of solutions to this problem, including the use of real time digital pre-processing of the image content to reduce ghosting in image. In particular, a digital processor applies a crosstalk model to predict potential ghosting by comparing the left- and right-eye images, and then it subtracts the predicted ghost image. U.S. Patent publication 2006/0268104 to M. Cowan et al., entitled “Ghost-compensation for improved stereoscopic projection”, expands upon this approach. As another example, in U.S. Pat. No. 7,518,662 by Chen et al., the polarization contrast of the ZScreen switcher is improved with a tilted polarization compensator.
For a variety of reasons, including improving light efficiency, expanding color gamut, increasing light source lifetime and reducing ongoing replacement costs, there is continuing impetus to replace the traditional lamps (such xenon arc, tungsten halogen, or UHP) with solid state light sources (such as lasers or LEDs) in projectors, whether 2-D or 3-D. However, to date, the desire for laser-based projection systems has been unfulfilled, in part as compact, robust, low-to-moderate cost, visible wavelength laser technologies had not emerged in a commercializable form, particularly for green and blue. With the recent emergence of blue diode lasers and compact green SHG lasers, low cost, laser based, pico-projectors from companies such as Microvision are reaching the market place.
In parallel, similar obstacles for compact high power visible lasers capable of supporting digital cinema projection have also started to disappear, as companies such as Laser Light Engines (Salem, N.H.) and Necsel (Milpitas, Calif.) have demonstrated prototype or early product laser devices. For example, Necsel (previously known as Novalux) offers green (532 nm) and blue (465 nm) laser arrays, each of which provides 3-5 Watts of optical output power. At these power levels, and allowing for system efficiency losses, a modest sized projector (˜1500 lumens output) for a large conference room or a home theatre, can be achieved using a single laser device per color. However, in the case of cinema, the on-screen luminance requires 10,000-40,000 lumens or 40-170 Watts of combined optical power (flux) incident to the screen, depending on screen size and screen gain. In turn, allowing for internal optical efficiency losses, this means that 40-120 Watts of optical power is required from the laser sources in each color channel. Presently, these power levels can only be accomplished by optically combining the output of multiple laser arrays in each color channel. Eventually, the laser technologies may advance such that a single compact laser device can drive each color. Of course, each approach has its advantages and disadvantages, relative to trade-offs of simplicity, cost, and susceptibility to laser failure.
Simplistically, a digital cinema projector can be provided by replacing the conventional lamp used for the light source 50 of FIG. 1A with a multitude of laser devices. Moreover, as lasers are inherently polarized light sources, more efficient 3D projection can be provided, as the polarization switcher is used at position 90, without an accompanying polarizer. However, this simplistic view is unrealistic for high power laser-based projection applications such as for digital cinema. As just suggested, a projection system providing 40-170 optical watts on screen is subjected to much higher light levels internally, with the highest light levels (in flux or watts) occurring at the light source assemblies, and the lowest likely at the output surface of the projection lens. Because of its spatial and temporal coherence, laser light focuses into smaller volumes with higher power densities than light beams from incoherent (lamp) sources, even when flux levels are comparable. The highest internal power densities occur in places where the light is concentrated, such as at an integrating bar, the spatial light modulators, aperture stops, or intermediate images. Of course, these high light levels can bring accompanying thermal issues, as illumination light, imaging light, or even stray light encounters internal surfaces or materials.
There are already numerous problems caused by the intense light in conventional lamp based systems, some of which will only be amplified in laser-based systems. For example, in a lamp-based system, the input aperture of the integrating bar, which receives high intensity focused light and surrounding stray light from the lamp, is typically surrounded with an air-cooled heat sink assembly. As another example, in digital cinema projection systems, the spatial light modulators are typically cooled with circulating chilled water.
At such high light levels, the intense light (and particularly residual UV light) can also impact the performance or reliability of materials, including optical adhesives, cements, or polymers used in prism elements, doublets, or liquid crystal devices. As a result, these materials must be chosen carefully to avoid induced degradation from thermal or chemical changes. Likewise, induced mechanical stresses from mismatched coefficients of thermal expansion of the optical elements or their mounting assemblies must also be minimized or managed to avoid stress, deformation, or breakage.
As one particularly subtle effect, which effects polarization based projection systems, including those for 3D projection, small portions of the high light intensity light can be absorbed by the optical materials, thereby inducing stress birefringence with the elements. That in turn can change the polarization orientations of the transiting light, thereby impacting image contrast, image contrast uniformity, color uniformity or other attributes that reduce the perceived on-screen image quality.
In the case of spatial light modulator devices, and liquid crystal on silicon (LCOS) devices in particular, a problem can occur where the intense light causes thermal loading and stress birefringence in the counter electrode substrate, which is internal to the device itself. To give further context, FIG. 1B illustrates a prior art projector 101, in which incident illumination light beams 140 are directed into respective modulation optical systems 80 for each color, which are projector sub-systems that comprise a polarization beamsplitter 60 (also known as a polarization prism), a polarization compensator 360, and a spatial light modulator 170g, 170b or 170r. The modulated beams from the modulation optical systems 80 are combined using an X-prism 65, and directed to projection lens 270 for projection onto a display screen (not shown). Typically, the polarization behavior and properties of these components within the modulation optical system 80 determines the on-screen polarization contrast provided by the projector 101.
The counter electrode substrate (not shown) is a thin plate of optical glass that is laid parallel to the silicon substrate within LCD spatial light modulators 170g, 170b and 170r. Liquid crystal materials, as well as pixel structures formed into (or on) the silicon, then fill the thin gap between these substrates. The counter electrode substrate is coated with a patterned transparent electrode (typically of ITO), to enable electric fields to be applied between the substrates to control the orientations of the liquid crystal molecules on a pixel wise basis.
This structure works well at low light intensities, such that the polarization orientations commanded by the pixels are maintained as the light transits the counter electrode substrate, and the resulting polarized image light can then encounter downstream polarization optics, such as polarization beam splitters, analyzers, or switches, with the polarized image light having the intended orientations. However, under high light intensities, the portion of the light transiting the counter electrode substrate that is absorbed can cause sufficient internal heating to induce stress birefringence, which in turn alters polarization orientations.
In recognition of this problem, U.S. Pat. No. 5,576,854 to Schmidt et al., proposes a method for identifying optimal glasses that can be used to fabricate the counter electrode substrate of an LCOS panel. In particular, they proposed a figure of merit M for identifying candidate glasses given by the product:M=ρEκ  (1)where ρ is the coefficient of thermal expansion (CTE), κ is the stress optical coefficient, and E is the modulus of elasticity (E). Schmidt et al. identified two glasses as particularly valuable candidates; Schott SF-57 for its unusually low stress optical coefficient, and fused silica for its unusually low coefficient of thermal expansion. According to Schmidt et al., in the case of fused silica, heating causes minimal expansion of the glass, which in turn cause little thermally induced stress. In the case of SF-57, the thermal stress coefficient itself is very low, which means little direct translation of heat into stress birefringence. As alluded to earlier, a similar problem presently exists with the cover glass windows for DLP modulators; but as these devices have generally not been used to modulate intense polarized light with an expectation of maintaining polarization states, neither the glass selection nor the glass mounting design were undertaken with the goal of minimizing stress birefringence.
The relationship of glass selection and thermal stress birefringence in projection displays is also explored in the article “Thermal Stress Birefringence in LCOS Projection Displays”, by R. Cline et al., which was published in Displays, Vol. 23, pp. 151-159, 2002. This article is concerned with identifying glasses appropriate for use in polarization beamsplitters 60 (FIG. 1B) or Philips prism assemblies 55 (FIG. 1A) in projection display systems. In particular, the authors introduce an expanded figure of merit for assessing candidate glasses that includes not only the coefficient of thermal expansion (p), the stress optical coefficient (κ), and the modulus of elasticity (E), but also the glass thermal conductivity (K), light absorption (α), and Poisson's ratio (μ):
                    M        =                              α            ⁢                                                  ⁢            ρ            ⁢                                                  ⁢            E            ⁢                                                  ⁢            κ                                K            ⁡                          (                              1                -                μ                            )                                                          (        2        )            Cline et al. propose that only Schott SF-57, Ohara PBH56, and fused silica can be used in prisms for high power polarization sensitive projectors (1000+ lumens), while a wider range of glasses, including Schott SK5 or Schott BK7, can be used for prisms in low power (≦500 lumen) projectors.
In contrast, in U.S. Pat. No. 7,357,511 to Aastuen et al., the inventors suggest that the glasses proposed by Cline et al., for satisfactory low stress birefringence (such as Schott SK5 or Schott BK7) are actually inadequate, and that contrast degradation from these alternate glasses is actually too large. Aastuen et al. then propose an alternate modulation optical system 80 where the polarization contrast of the polarization beamsplitter 60 can be improved relative to stress birefringence in the glass comprising the prism, including thermally induced stress birefringence, by providing a polarization compensator 360 between the polarization beamsplitter 60 and the spatial light modulator 170 (see FIG. 1B). They provide evidence that a polarization compensator 360 having a quarter wave of retardance can provide sufficient compensation for stress birefringence such that the prism glass choice is no longer limited to low stress optical coefficient (κ) glasses, such as Schott SF-57.
It is also noted that unwanted birefringence has caused image quality problems in fields outside of the projection space, including in the area of micro-lithography. For example, in U.S. Pat. No. 6,785,051 to Allan et al., describes a refractive/reflective imaging system directed at 200 nm UV microlithography. In that spectral range, the very small selection of available optical materials is dominated by crystalline materials such as calcium fluoride (CaF2) that exhibit significant intrinsic birefringence. In order to reduce the accumulative birefringence or polarization state changes in the optics, Allan et al. provide one or more corrective optical elements (optical plates or beam-splitters) which are also fabricated from the same type of intrinsically birefringent materials. In this case, a corrective photoelastic birefringence is provided by an externally applied stress or strain (from tensile, compressive or shear stress) which was imparted to the corrective element by mechanical fixturing, a piezoelectric actuator, a thermal element or other stress inducer.
Likewise, U.S. Pat. No. 6,879,379 to Brunotte et al., also discloses a UV microlithographic imaging system using lens elements comprising intrinsically birefringent materials such as CaF2 and a corrective element. The intrinsic birefringence imparts unwanted polarization rotation effects with position and angle. In this case, the corrective element is an optical plate or lens which is located proximate to an aperture stop, and which is also made from CaF2. Mechanical stresses are then applied in a pulsed fashion using piezoelectric actuators, so as to impart stress birefringence to the element that compensates for the angle dependent polarization effects caused by the intrinsic birefringence.
While interesting, the solutions of Brunotte et al. ('379) and Allan et al. ('051) apply to imaging systems using a limited set of intrinsically birefringent materials. By comparison, the solutions provided by Schmidt et al. ('854), Cline et al., and Aastuen et al. ('511) were developed in the context of lamp-based projection systems that were targeted for low power applications, but are potentially extendable to digital cinema. However, these solutions are narrowly targeted at the optical components (cover glasses and prisms) within the modulation optical systems 80 for a projector 101.
In laser projection systems the localized light intensities and power densities can be appreciably higher as compared to white-light systems, due to the coherence or focusing power of the laser light, and thermal effects can be induced throughout an optical system. In extreme cases, optical self-focusing effects in non-linear optical materials can cause optical damage or breakdown.
In the case of laser-based digital cinema projectors, while permanent damage mechanisms such as self-focusing are likely not germane, other thermal effects such as thermally induced stress birefringence can affect optical elements, including components other than those residing in the modulation optical subsystems, such as the prism assemblies, spatial light modulators, or cover plates or counter electrode substrate therein. In particular, the design and use of imaging lens assemblies, which comprise a complex multitude of lens elements, and which are tasked to image intense laser light while not being subject to thermally induced stress birefringence and resulting polarization effects, is a concern, particularly at the digital cinema power levels, which are much higher than managed previously for stress birefringence. As is known to those skilled in the lens design arts, an imaging lens assembly utilizes an arrangement of non-planar lens elements, whose materials, thicknesses, curvatures, and relative placements are carefully designed to provide the desired image quality, relative to aberrations and diffraction. However, the added complexity of further controlling thermally induced stress birefringence, relative to the design of an imaging lens system and the constituent lens elements, is a problem that is neither taught nor anticipated in the prior art.